This invention relates generally to improvements in controlling growth processes of single crystal semiconductors for use in the manufacture of electronic components and, particularly, to a method and apparatus for accurately controlling the diameter of a single crystal silicon ingot being pulled from a semiconductor source melt according to a predetermined velocity profile.
Monocrystalline, or single crystal, silicon is the starting material in most processes for fabricating semiconductor electronic components. Crystal pulling machines employing the Czochralski crystal growth process produce the majority of single crystal silicon. Briefly described, the Czochralski process involves melting a charge of high-purity polycrystalline silicon in a quartz crucible located in a specifically designed furnace. After the heated crucible melts the silicon charge, a crystal lifting mechanism lowers a seed crystal into contact with the molten silicon. The mechanism then withdraws the seed to pull a growing crystal from the silicon melt. A typical crystal lifting mechanism suspends the seed crystal from one end of a cable, the other end of which is wrapped around a drum. As the drum rotates, the seed crystal moves up or down depending on the direction that the drum is rotating.
After formation of a crystal neck, the growth process enlarges the diameter of the growing crystal by decreasing the pulling rate and/or the melt temperature until a desired diameter is reached. By controlling the pull rate and the melt temperature while compensating for the decreasing melt level, the main body of the crystal is grown so that it has an approximately constant diameter (i.e., it is generally cylindrical). Near the end of the growth process but before the crucible is emptied of molten silicon, the process gradually reduces the crystal diameter to form an end cone. Typically, the end cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt. During the growth process, the crucible rotates the melt in one direction and the crystal lifting mechanism rotates its pulling cable, or shaft, along with the seed and the crystal, in an opposite direction.
Although presently available Czochralski growth processes have been satisfactory for growing single crystal silicon useful in a wide variety of applications, further improvements are still desired. For example, a number of defects in single crystal silicon form in the crystal growth chamber as the crystal ingot cools after solidification. Such defects arise, in part, due to the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects known as vacancies and self-interstitials. It has been suggested that the type and initial concentration of these point defects in the silicon can influence the type and presence of agglomerated defects in the final product. If these concentrations reach a level of critical supersaturation in the system and the mobility of the point defects is sufficiently high, a reaction, or an agglomeration event, will likely occur. Agglomerated intrinsic point defects in silicon can severely impact the yield potential of the material in the production of complex integrated circuits.
Accurately pulling a single crystal silicon ingot from a melt according to a predetermined velocity profile, or target, specified in a crystal xe2x80x9crecipexe2x80x9d helps satisfy process needs for controlling the formation of defects. For example, this type of control (herein referred to as a xe2x80x9clocked seed liftxe2x80x9d process) reduces the number and concentration of intrinsic point defects in the ingot. In addition, a locked seed lift process helps control the concentration of vacancies and self-interstitials to prevent an agglomeration of intrinsic point defects in the ingot as the ingot cools from the solidification temperature. Conventional Czochralski silicon growth processes, however, vary the pull rate, or seed lift, to control the diameter of the growing crystal. Those skilled in the art recognize that increasing pull rate causes a reduction in crystal diameter while decreasing the pull rate causes an increase in diameter. It is also well known that increasing the temperature of the silicon source melt causes a reduction in crystal diameter while decreasing the melt temperature causes an increase in diameter. For these reasons, controlling the pull rate according to a target profile can lead to diameter errors unless the melt temperature is accurately adjusted during pulling.
Unfortunately, using the pull rate to control crystal diameter is generally preferred in conventional growth processes because the delay in effecting melt temperature changes is usually unacceptable. In other words, the selection of pull rate instead of temperature to control diameter is based on the difference in response times, the response time for temperature changes being much slower than the response time for pull rate changes. For example, a step change in pull rate typically achieves a diameter response in seconds whereas a step change in heater power or melt temperature results in a much more sluggish response taking tens of minutes to achieve an equivalent effect.
For these reasons, an accurate and reliable apparatus and method for pulling a single crystal silicon ingot from a melt is desired for controlling the diameter of a silicon crystal using only heater power and eliminating the pull rate variability typically required to control diameter.
The invention meets the above needs and overcomes the deficiencies of the prior art by providing a method and apparatus for adjusting power for maintaining adequate control of crystal diameter in a locked seed lift process. Among the several objects of the invention may be noted the provision of a method and apparatus that provides accurate diameter control; the provision of such method and apparatus that adjusts the crystal diameter by changing the melt temperature; the provision of such method and apparatus that provides relatively fast melt temperature changes; the provision of such method and apparatus that permits modeling the temperature response of the melt; the provision of such method and apparatus that permits changing the crystal diameter as a function of heater power, the provision of such method and apparatus that may be incorporated into existing crystal pulling devices; and the provision of such method and apparatus that is economically feasible and commercially practical.
Briefly described, a method embodying aspects of the invention is for use in combination with an apparatus for growing a monocrystalline ingot according to the Czochralski process. The apparatus has a heated crucible containing a semiconductor melt from which the ingot is grown on a seed crystal that is pulled from the melt. The method includes pulling the ingot from the melt at a target rate that substantially follows a predetermined velocity profile. The method also includes the step of defining a temperature model representative of variations in the temperature of the melt in response to variations in power supplied to a heater for heating the melt. In generating a temperature set point representing a target melt temperature, the method next includes the steps of generating a signal representative of an error between a target diameter and a measured diameter of the ingot, performing proportional-integral-derivative (PID) control on the error signal and generating the temperature set point as a function thereof The method further includes determining a power set point for the power supplied to the heater from the temperature model as a function of the temperature set point generated by the PID control and adjusting the power supplied to the heater according to the power set point. In this manner, the temperature of the melt is changed for controlling the diameter of the ingot.
Another embodiment of the invention is directed to an apparatus for use in combination with an apparatus for growing a monocrystalline ingot according to the Czochralski process. The apparatus has a heated crucible containing a semiconductor melt from which the ingot is grown on a seed crystal that is pulled from the melt. The apparatus includes a predetermined velocity profile and the ingot is pulled from the melt at a target rate substantially following the velocity profile. The apparatus also includes a PID control generating a temperature set point as a function of an error between a target diameter and a measured diameter of the ingot. The temperature set point represents a target temperature of the melt. A temperature model represents variations in the temperature of the melt in response to variations in power supplied to a heater for heating the melt. The temperature model determines a power set point for the power supplied to the heater as a function of the temperature set point generated by the PID control. The apparatus further includes a heater for heating the melt and a power supply responsive to the power set point for adjusting the power applied to the heater. In this manner, the apparatus changes the temperature of the melt to control the diameter of the ingot.
Alternatively, the invention may comprise various other methods and apparatuss.
Other objects and features will be in part apparent and in part pointed out hereinafter.